Synchronous electric motors in motorized fan units require to be of high power, and to give high outputs. Because such motors are fitted in confined spaces under the hood of the vehicle, it is not possible to allow such motors or their control means to emit unduly high amounts of heat. It is therefore, very desirable to obtain a considerable reduction in heat loss in such motors.
It is known that the mechanical power P.sub.Mec of an electric motor is given by the expression P.sub.Mec =E.I, and that the energy losses P.sub.T these are given by the expression P.sub.T =RI.sup.2, where E is a vector the different components of which correspond to the counter-electromotive forces in the various windings of the armature of the motor; and where I is a vector the different components of which correspond to the current intensities in the different windings. The expression E.I is given by: ##EQU1## where n is the number of phases in the motor and T is the period of the control signals.
For a given power P.sub.mec, the losses P.sub.T are minimised if E and I are proportional to each other.
One solution which a person skilled in the art could consider in rendering the vectors E and I proportional to each other, would be to apply to the various windings supply currents which follow predetermined laws of variation analagous to those of the counter-electromotive forces which are known beforehand, and to synchronise this current supply with the rotation of the rotor.
However, this is not an optimum solution. The laws of variation of the counter-electromotive forces change significantly over time, due in particular to the thermal effects of the ferrites of the motor, or again to the reactions in the armature, which distort these counter-electromotive forces. These variation laws also vary significantly from one motor to another, or even from one winding to another. As a result of this, the theoretical laws of variation that could be imposed on the currents in the windings can only be appoximate, and can only lead to the heat losses not being optimised.
In addition, such a solution would have the disadvantage of requiring an electronic synchronising unit to be provided.